1. Field of the Invention
The present invention relates to an electrostatic image forming apparatus, and more particularly to a charge control device for the photosensitive member in the image forming apparatus.
2. Description of the Prior Art
The means for image formation by the electrostatic method, in which an electrostatic latent image is formed on a photosensitive member, the image is developed with toner and then transferred to recording paper and fixed thereon, and a visible image is thereby produced, is widely employed for copying machines, laser printers, and so on.
In such electrostatic image forming means, if electrical charging is applied to the photosensitive member in reverse polarity to that of the main charging therefor which depends on the characteristic of the photosensitive member (the main charging is meant the charging of negative polarity when the photosensitive member is of P-type semiconductor, or the charging of positive polarity when the photosensitive member is of N-type semiconductor), then, it causes difficulty in applying the main charges onto the photosensitive member in the charging operation of next time, unless the charges are erased in a short time. The difficulty is likely to produce bad effects such that fogs or the like appear in the formed image, as will be discussed later, when reverse development is practiced.
In the case of reproduction of a positive image from an original of a positive image, i.e., the positive-to-positive image formation, as is the case with the ordinary electrophotographic copiers, such procedure is taken that electrical charges for the main charging are first applied onto the photosensitive member, the same is exposed to the optical image from the original and an electrostatic latent image is thereby formed on the same, the electrostatic latent image is then developed with toner having electrical charges of reverse polarity to that of the charges for the main charging, and then the toner image is transferred onto recording paper while charges of the same polarity to that of the charges for the main charging are given thereto by a transferring charger.
Therefore, charges of the reverse polarity will never be left on the photosensitive member and hence any particular problem is caused.
However, if the same steps as described above are taken in the case of forming a positive image on recording paper from the image exposed on a photosensitive member by laser beam or forming a positive image reproduction from a microfilm having a negative image, the toner will not adhere to the exposed portion of the photosensitive member but rather adheres to the unexposed portion thereof. Accordingly the obtained image transferred onto recording paper becomes a negative image.
In a negative-to-positive image formation to provide a positive image from such a negative image, although the steps from charging the photosensitive member up to exposing to the optical image are conducted in the same way as in the above described positive-to-positive image formation, toner with charges of the same polarity as that of the charges for the main charging is used for development of the electrostatic latent image, whereby the toner is adhered to the portion that has been exposed to the optical image. And the toner image is transferred onto recording paper while the charge of the reverse polarity to the main charging is provided by a transfer charger.
Thus, it sometimes occurs that electrical charges of the reverse polarity to that of the charges for the main charging are applied to the photosensitive member from the transfer charger at the time of the image transfer, and this causes fogs or the like in the picture image. More specifically, the portion applied with the charges of the reverse polarity to that of the main charging will not reach a specific charged level when the same is subjected to the next main charging, and therefore, the toner tends to adhere this portion on account of the development bias voltage in the reverse development thus causing fogs in the picture image.
To cope with the above mentioned problem, measures such as AC charge-elimination have so far been adopted, in which AC charges are applied to the photosensitive member inclusive of the recording paper immediately after a toner image has been transferred to recording paper thereby to remove the electrical charges. By taking such measures the charges on the photosensitive member are virtually neutralized, or brought to the same polarity as that of the main charging, so that adverse influence is not produced at the time of the image formation next time.
When the image forming operation has been finished and the rotation of the photosensitive member is stopped, the burden of the photosensitive member is lightened by eliminating the charges on the photosensitive member by such means as light erasing to bring its surface potential virtually to zero potential.
In that case, if there are left any small amount of charges, which are of the reverse polarity as that of the main charging, they cause lowering of the charged potential when the photosensitive member is charged in the image forming operation next time.
In the image forming apparatus of this type in general, the transfer charger for transferring a toner image on the photosensitive member to recording paper and the AC charge-eliminator which becomes operative immediately after the transfer has been made are integrally formed side by side and their operations are controlled as if they were an integral body. Therefore, when the photosensitive member stops its rotation after the image forming operation has been finished, both the transfer charger and the AC charge-eliminator stop their operations simultaneously.
Therefore, when the photosensitive member stops its operation, there remains on the photosensitive member such a portion that is charged subjected to the action of the transfer charger and yet not subjected to the action of the AC charge-eliminator, and from this, it follows that some charges of the reverse polarity to the main charging remain locally on the photosensitive member.
In order to eliminate such residual charges, it has so far been required to subject the photosensitive member to charging by the main charger and to light erasing by rotating the photosensitive member additional turns after the image forming operation has been finished--the operation being the rotations of the photosensitive member after the completion of the image formation is called the "postrotation". Also, in an image forming apparatus including no AC charge-eliminator, the practice of the post-rotation has been required to eliminate the residual charges.
However, since the post-rotation causes wear of the photosensitive member as well as decrease and fatigue of the photosensitivity of the photosensitive member and therefore shortens the life of the photosensitive member, it has been desired that the post-rotation may be reduced as much as possible.